The invention pertains to a solid-state imaging device and a method for operating the imaging device that provides wide dynamic range and multiple gray-scale image signals.
Solid-state imaging devices using charge-coupled devices (xe2x80x9cCCDsxe2x80x9d) have been used as image sensors in electronic and video cameras. Solid-state imaging devices have been further developed to be sensitive to infrared radiation for applications such as thermal imaging or night vision.
With reference to FIG. 15, a prior-art solid-state imaging device for infrared imaging uses a two-dimensional array of PtSi Schottky diodes 61. xe2x80x9cVerticalxe2x80x9d CCD arrays 62 are provided adjacent the columns of the PtSi diodes 61 and columnar transfer gates 63 are placed between respective vertical CCD arrays and columns of PtSi diodes 61.
Electrodes xcfx861-xcfx864 are connected to the vertical CCD arrays 62. In FIG. 15, the electrodes xcfx861-xcfx864 are shown connected to one of the vertical CCD arrays 62 corresponding to the first column of PtSi diodes 61. For simplicity, the connections of the electrodes xcfx861-xcfx864 to the other vertical CCD arrays 62 are not shown. One end of each of the vertical CCD arrays 62 is connected to a xe2x80x9chorizontalxe2x80x9d CCD array 64. An output amplifier 66 is connected to an output buffer 64a of the horizontal CCD array 64. The other ends of the vetical CCD arrays 62 are connected to a discharge line 67.
The imaging device of FIG. 15 provides interlaced output by providing odd and even fields. An operation in which an odd field is read out is described below. First, voltages are applied to the electrodes xcfx861-xcfx863, forming potential wells at the vertical CCD arrays 62 that receive signal charges accumulated by the PtSi diodes 61. The accumulated charges are transferred to the potential wells with the transfer gates 63. Signal charges on an nth row (where n is an odd integer) of PtSi diodes 61 and signal charges on an (n+1)th row are mixed in one potential well. Thus, signal charges for a single horizontal line are produced. Next, four different driving pulses are applied successively to the electrodes xcfx861-xcfx864, transferring the signal charges in the potential wells vertically to the horizontal CCD array 64.
The horizontal CCD array 64 successively receives signal charges from rows of the diodes 61; the signal charges are delivered to the horizontal CCD array 64 from the vertical CCD arrays 62. These signal charges are transferred to the amplifier 66 during a horizontal scanning interval.
An even field is read by forming potential wells under the electrodes xcfx864, xcfx861, xcfx862. Signal charges on an nth row (where n is an odd integer) of PtSi diodes 61 and signal charges on an (nxe2x88x921)th row are mixed, whereby signal charges corresponding to a horizontal line are produced.
The readout operations are carried out so that signal charges for an even field or an odd field are read out sequentially during one field period, e.g. {fraction (1/60)} s for NTSC standard video. Readout of an odd field and an even field (one complete frame or image) requires {fraction (1/30)} s.
Imaging devices with electronic shutter capability (i.e. that have variable signal charge accumulation times) have been disclosed in Japanese Examined Patent Publication No. 1-18629. Electronic shutter operation can also be described with reference to FIG. 15. First, unwanted charges accumulated on the PtSi diodes 61 are transferred to the vertical CCD arrays 62 via the transfer gates 63. The vertical CCD arrays 62 are driven so that the unwanted charges are swept to the discharge line 67.
After a specified accumulation time has elapsed after the discharge at the discharge line 67, accumulated signal charges are transferred to the vertical CCD arrays 62 via the transfer gates 63. The vertical CCD arrays 62 are driven so that the signal charges from the rows of PtSi diodes 61 are sequentially transferred to the horizontal CCD array 64. The horizontal CCD array 64 then transfers the signal charges to the amplifier 66. By varying the accumulation time, an electronic shutter operation is achieved.
With reference to FIG. 16, the output of the imaging device of FIG. 15 with electronic shutter operation is shown as a function of blackbody temperature of an object being imaged. The blackbody temperatures were measured using a blackbody oven; such blackbody temperatures are analogous to luminance levels of visible light. The vertical axis indicates the number (in base-10 exponential notation) of output electrons generated by the PtSi Schottky diodes 61 for objects as a function of blackbody temperature.
With reference to FIG. 16, output-characteristic curves 16a-16d show outputs of the horizontal CCD array 64 for signal-charge accumulation times of {fraction (1/60)}, {fraction (1/500)}, {fraction (1/1000)}, and {fraction (1/1500)} s, respectively. For an accumulation time of {fraction (1/60)} s as shown by curve 16a, there is a large variation in charge output for objects with a range of blackbody temperatures from 20 C. to 80 C., and the images produced have a wide dynamic range. Objects associated with this range of blackbody temperatures are therefore imaged with multiple gray-scale levels. However, at blackbody temperature greater than about 100 C., signal charges fill and overflow the PtSi diodes 61, and the output saturates. Thus, images of objects at blackbody temperatures greater than 100 C. have on a few gray-scale levels.
With reference to curve 16d, corresponding to an accumulation time of {fraction (1/1500)} s, the output charge changes rapidly with temperature for blackbody temperatures near 200 C. Objects at blackbody temperatures near 200 C. are therefore imaged with multiple gray-scale levels, and excellent images are formed of such objects. However, at blackbody temperature near 20 C., the output is low, and the output signal-to-noise ratio is low. Images of objects at these temperatures are noisy and have few gray-scale levels.
The curves 16b, 16c, corresponding to signal charge accumulation times of {fraction (1/500)} s and {fraction (1/1000)} s, respectively, have narrow ranges of blackbody temperatures at which images with multiple gray-scale levels are produced. As will be readily apparent from curves 16a-16d, high-quality (i.e., wide dynamic range, multiple gray-scale) images are difficult to obtain of objects having large temperature variations.
The above description concerns exposure control with an electronic shutter, but a similar problem occurs with exposure control using lens apertures or filters. These exposure-control techniques also do not improve the dynamic range or the gray-scale output of the imaging device. While the description above concerns infrared imaging with infrared-sensitive imaging devices, it is similarly difficult for an imaging device sensitive to visible light to produce images with a wide dynamic range and multiple gray-scale levels.
It is apparent that imaging devices and methods are needed that produce images with a wide dynamic range and multiple gray-scale levels.
The invention provides solid-state imaging devices comprising a plurality of photosensors arranged in a matrix of rows and columns. The rows comprise odd-numbered rows and even-numbered rows. The photosensors accumulate signal charges corresponding to an incident light flux. xe2x80x9cVerticalxe2x80x9d transfer paths corresponding to each column of photosensors are provided to transfer signal charges (or unwanted charges) to a xe2x80x9chorizontalxe2x80x9d transfer path. The horizontal transfer path transfers the charges to an output amplifier that produces an electrical image signal corresponding to the distribution of the light flux intensity on the photosensors. Each of the vertical and horizontal transfer paths are preferably arrays of charged-coupled devices (CCDs).
Gates are provided to control the transfer of charges from the photosensors to the vertical transfer paths. The gates are controlled so that the accumulation times during which signal charges produced by the photosensors accumulate are adjustable. The gates preferably control accumulation times so that the odd rows and even rows have different accumulation times. The longer accumulation time is preferably set so that the photosensors that are irradiated by the most intense portion of the incident light flux are saturated. Such an imaging device produces image signals having a wide dynamic range and multiple gray-scale levels.
The photosensors are preferably PtSi Schottky diodes or semiconductor junctions, but it will be apparent that other photosensors are suitable. Depending on the photosensors, the imaging device is suitable for use in various wavelength ranges, including the visible and infrared ranges.
Shallow well junctions or lower potential barriers are preferably provided so that excess signal charge is discharged to the substrate of the imaging device. In this way, the excess charge does not reach other photosensors or the vertical transfer paths, and image blooming is suppressed. The potential-barrier height of the shallow well junctions or lower potential barriers is preferably shallower than the potential barriers of the p-n junction of the photosensor.
The imaging device can further comprise a summing junction that sums the signal charges of an odd row and an adjacent even row so that the charges from photosensors in the same column are summed. The summing can be done on the vertical transfer paths or on an external adder in conjunction with an external delay
The invention also provides methods of operating solid-state imaging devices so that wide dynamic range, multiple gray-scale images are formed. The methods comprise the step of accumulating signal charge on adjacent rows of photosensors for different accumulation times and then summing the signal charges from the corresponding columns of the adjacent rows. The charges can be summed in potential wells in the imaging device or summed externally. The summed charges are then delivered to an output amplifier. The different accumulation times are preferably associated with odd and even rows alternately. For example, in forming an interlaced scan image, the odd rows and the even rows have first and second accumulation times during an odd field; during an even field, the odd rows have the second accumulation time and the even rows have the first accumulation time. The longer of the first and second accumulation times is adjusted so that one or more photosensors saturate.
Because even and odd rows have different accumulation times, the imaging device provides an image with multiple gray-scale levels irrespective of whether the object imaged is bright or dark. Moreover, by selecting only the odd rows or the even rows, an image representing either a bright or dark area is produced with multiple gray-scale levels.
The accumulation times for odd and even rows need not be switched, but the following disadvantage is avoided by switching these times. For an odd field, charges on an nth row (where n is an odd integer) of photosensors and those on an (n+1)th row are summed. In conventional two-line interlaced scanning, the signal charges on the nth and (n+1)th row are approximately equal. The apparent center of a horizontal line made by the summation is therefore nearly equidistant from the nth row and the (n+1)th row. However, in the imaging devices of the present invention, the accumulation times. differ for even and odd rows of photosensors. The apparent center of a horizontal line corresponding to the summed charges is nearer the row having the longer accumulation time.
In an even field, charges on an (nxe2x88x921)th and an nth row of photosensors are summed. In this case as well, the apparent center of such a horizontal line is nearer the row having the longer accumulation time. The displacement of the apparent centers degrades image resolution. In a display device such as a shadow-mask cathode-ray tube, horizontal lines corresponding to even and odd fields are uniformly spaced irrespective of the shift of the apparent center. By alternately switching accumulation times between odd and even fields, the apparent centers are not displaced.
In addition, accumulation times can be alternated between even and odd fields, whereby the odd field accumulates charge for a first accumulation time, and the even field accumulates charge for a second accumulation time. The accumulation times are then switched so that the next odd field accumulates charge for the second accumulation time, and the next even field accumulates charge for the first accumulation time.
The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description of Example Embodiments which proceeds with reference to the accompanying drawings.